Microwave power amplifiers, boosting the radio signal to a sufficient power level for transmission through the air interface from the transmitter to the receiver, are important components in wireless communications systems. They are the circuits that convert DC power into RF/microwave output power and in addition power amplifiers consume a significant amount of power, especially in portable devices. Inherently, the microwave transistors of the power amplifiers are nonlinear. Intermodulation distortion (IMD) is a common problem suffered from high-efficiency amplification since the amplifier is operating within its nonlinear region. Its products cause both in-band distortion and out-of-band emission. To keep the adjacent channel interference tightly within the specification of the systems, backing-off the output power of the amplifier is the simplest solution. However, this results in degradation of efficiency. Therefore, linearity and efficiency are highly desirable objectives of power amplifier designs.
In future wireless communications systems, non-constant envelope modulation schemes, such as M-PSK (Multiple Phase Shift Keying) and QAM (Quadrature Amplitude Modulation), are anticipated to accommodate higher channel capacity and better bandwidth efficiency in wireless transmission. The information is carried in such digital modulations by both amplitude and phase of the RF signals, therefore, highly linear amplification is desirable to meet the stringent requirements on both transmitted spectrum emission and received signal-to-noise ratio. Class A and Class AB power amplifiers are potential candidates, but they suffer the problem of low average power efficiency. Therefore, low distortion and high average power efficiency are the key issues for the next-generation power amplifier design.
In view of compensating the tradeoffs between linearity and efficiency of power amplifiers, various linearizing and bias controlling circuitries have been explored.
System-level linearization techniques, like feedforward, digital predistortion and even-order signals injection, provide excellent suppressions of spectral regrowth. However, these techniques require complicated and expensive circuitry, additional power consumption and large occupation of printed circuit broad area. As a result, only base station applications can benefit from these techniques.
Circuit-level techniques utilize the nonlinear characteristic of microwave diodes or transistors, which compensate for the nonlinear variation of the internal components of the amplifying transistors or distort the signals before or after the amplifying transistors, provide a compact and low-cost approach for the handset applications. Examples of such prior art, utilizing a single microwave diode, are shown in FIG. 1, FIG. 2 and FIG. 3.
The methodology of the approach in FIG. 1 is to compensate for the nonlinear capacitor, typically the base-collector capacitor (CBC) of bipolar transistors and the gate-source capacitor (CGS) of field-effect transistors for compensating the phase distortion (amplitude modulation to phase modulation distortion) of the amplifier. The methodology utilizes the nonlinear capacitor of a reverse-biased microwave diode (i.e. VL is positive) to provide a substantially constant of resultant capacitance at the input terminal of the amplifying transistor.
The methodology of the approach in FIG. 2 is to compensate the nonlinear transconductance (gm) of bipolar transistors for compensating the gain compression of the amplifier. A nonlinear rectified current is present when there is RF signal passing through the diode (QL). Consequently, this rectified current increases with the increase of the input power. As a result, the junction voltage (VL) drops which leads to an increase of the base-emitter voltage (VBE) of the amplifying transistor.
The methodology of the approach in FIG. 3 is to distort the input signal with positive gain and negative phase deviations for compensating both the gain compression and phase distortion of the amplifier. A nonlinear rectified current is present when there is RF signal passing through the diode (QL). Consequently, this rectified current increases with the increase of the input power. As a result, the junction voltage (VL) drops which leads to the increase of the internal resistance of QL and the magnitude and the phase of the transfer function from RF input port (RFIN) to power amplifier input port (PAIN) are increased and decreased, respectively.
Inevitably, the nonlinearity matching between diode linearizers and amplifying transistors is rigid, and hence, the linearity improvement is limited.